The door to the fitness world's possible next craze is barely marked.

I look hard for a number, some signal to indicate I’m at the right address, on Lexington Ave. in east midtown Manhattan. Via email, I’ve been directed to head over and get the lowdown at the place called “the dungeon.”

That’s the test facility where the beyond-credible crew of entrepreneurial badasses is forging Rumble, a brand-new group boxing workout they believe will prove irresistible for people looking to get the drool-worthy bodies seen on the best boxers in the world. If they're right—and by all indications, they're on to something—Rumble could very well redefine how people get shredded.

Once I finally find the dungeon (metal door with brick stuck in it, then down a flight of stairs) I discover a bare-bones space filled with a crew of fitness freaks and low-body-fat fanatics with game faces on. Eugene Remm greets me. I’ve heard of him: Labeled a "king of New York hospitality," he has a reputation for creating hotspots like CATCH, a seafood joint making its mark in Dubai, Playa del Carmen, Los Angeles, and Manhattan's Meatpacking District. Remm is all casual intensity, welcoming but clearly dialed-in, the kind of dude who isn’t wasting time or energy on a concept that won’t have a high likelihood of spectacular success. He's a co-founder of Rumble, along with three similarly accomplished guys: Noah Neiman (an ex-Barry’s Bootcamp Master Trainer), Andy Stenzler (co-founder of Cosi, Kidville) and Anthony DiMarco (an Ironman vet, and former Google exec).

A crowd of male and female millennial types, aspiring trainers who are learning the Rumble way, look up from their phones and nod at me. The mood is like that of a locker room before an early-round playoff game, when defeat wouldn't be agonizing so much as embarrassing. (The “dungeon” isn't permanent, by the way. Rumble's first location opened on January 9 in Manhattan's Chelsea neighborhood, with another in NoHo set to open in spring of 2017. But for now, the space where we're working out—much like the Rumble workout itself—is a work in progress.)

The trainers are summoned down some dusty steps, past 20 pairs of boxing gloves, onto a gym floor, with grade-A speakers ready to make fat cells vibrate angrily with uplifting decibels. There are a bunch of specialized punching bags filled with water, Remm explains, to lessen the shock on the joints. The trainers, joined by some front desk personnel seeking a better sense of the workout, do stretches and as the group leader points them to different stations. Not that they're all complete novices: "See the gentlemen with the dreadlocks?" Remm says, gesturing to a ripped dude warming up. "He’s a professional fighter."

The workout begins, and the lead trainer, a woman who exudes magnetic leadership, starts cajoling the class through their 10-round, 45-minute session. “We’re certainly going to make the attempt to lead the field in group fitness boxing, absolutely,” Remm tells me. By the sound of it, they'll pull in some top-quality DJs, too—mixes and hip-hop and mashups will have the patrons feeling like mini-Balboas of the new millennium.

As for the workout itself? People will master six punches, Remm continues, and KO two goals with one workout: They’ll get a kickass core and the lean look that’s in vogue, while building up some basics that will boost their brains, not just bods. “We the balance the floor and the bag, which most gyms don’t do. Some people just have bags which you punch for 45 minutes. We also have weights and benches.” 

But while the workout might sound like CrossFit with boxing, the facilities sound, well, palatial: “The cleanliness of the property is nothing you’d think of 'boxing,'" Remm says, while closely watching one of the wannabe trainers who might not have the showmanship quotient Remm craves.

Neiman, the ex-Barry's Bootcamp guy, has that showmanship. Neiman shakes my hand, and I quickly look up his Twitter feed while he checks in with one of the female trainers (who, by the way, happens to be ex-WWE Diva Erika Hammond). “RUMBLE…for ppl who let their actions not only speak, but scream,” is Neiman's pinned Tweet. 

Neiman lets me in on a little background. Two years ago, Neiman says, Remm said he wanted to get into the fitness space. Barry's Bootcamp was hot, and Neiman thought boxing was going to be the next Sriracha. He’d been doing jiu-jitsu, then boxing, for a spell, and threw in with Remm, who brought him to touch base with Stenzler and DiMarco.

“We breathe this, seven days a week,” Neiman says. And why boxing? “I train because I love to feel empowered by what I do,” he says, recalling he used to train to get body beautiful, and then figured he wanted to instead “train for a feeling. And boxing made me feel fucking awesome.”

Stenzler, a serial entrepreneur, sidles over to put this group spin on how Rumble is changing the fight game. “In the past, when people thought of boxing, they conjured Rocky Balboa or George Foreman, in a dirty, dungeon gym," he says. "Now, though, it’s starting to be adopted. There’s so many people realizing that boxing is an amazing full-body workout. It’s not about getting hit, or being in the ring. At Rumble it’s about the fitness element, and sure, learning a skill…but at the end of the day it’s the combination of the energy, the excitement, the ability to hit something, and get some of those endorphins on the bag. Before this, nobody had really said, 'We’re going to take boxing to group fitness, and add a special, hospitality level to it.' All of group fitness can be elevated— and boxing can be the medium, because we love it.”

Hammond, who was a former WWE NXT wrestler before becoming one of Rumble's founding trainers, sauntered over. In the wrassling world, she was “Veronica Lane,” a snotty beauty queen character, but after a knee surgery she exited and did personal training; now she’s all-in with Rumble. She notes that a lot of Victoria’s Secrets models—Adriana Lima and the like—are espousing the boxing workout, the implication that men and women will both be moths to the Rumble-lit flame.

After an hour and a half in the dungeon, I'm starting to see how the boxing class could become a bona-fide craze by the end of 2017. I had been asked two, then three times in that span if I want to hop in for a few rounds. I politely decline; my attire isn’t right. But as I say my goodbyes and then exit the dungeon, I am throwing a hook at the air, then a right cross.

Maybe no workout for me today. But they may entice me yet.